A maleimide compound is a compound useful as an ingredient of resin, an ingredient of pharmaceuticals and pesticides, and the like, and often used as one of copolymerization components for improvement in heat resistance of, particularly, styrene-based resins such as ABS resins, AS resins, AB resins, ACS resins, AES resins and AAS resins, and polyvinyl chloride resins, polymethyl methacrylate resins, phenol resins, and the like. Among them, N-phenylmaleimide (hereinafter also referred to as PMI) is excellent in reactivity and heat resistance, and thus especially widely used.
As the method for producing a maleimide compound, many methods such as a method of obtaining by dehydrating maleic anhydride and a primary amine in one step (for example, Patent Literatures 1 and 2), a method of producing a maleamic acid from maleic anhydride and a primary amine and obtaining by a dehydration ring-closure imidization reaction of the maleamic acid (for example, Patent Literatures 3 to 6) and a method of obtaining by a ring-closure imidization reaction of a corresponding maleamic monoester (for example, Patent Literatures 7 to 9) are conventionally known. Among these methods, the method of obtaining by dehydrating maleic anhydride and a primary amine in one step has a problem that the yield is still low and thus the productivity is low, and the method of obtaining from a maleamic monoester has a problem that an alcohol generated by the ring-closure imidization remains and contaminates in the product, thus the method of obtaining by the dehydration ring-closure imidization reaction of a maleamic acid is generally industrially carried out. Here, when producing N-phenylmaleimide, the primary amine is aniline, and the maleamic acid is N-phenylmaleamic acid.
Incidentally, when producing N-phenylmaleimide according to the above method, it goes through the generation of N-phenylmaleamic acid (hereinafter also referred to as PMA) as an intermediate, and when this PMA is hydrolyzed, aniline and a maleic acid are produced. It is known that the produced aniline (or aniline derived from raw materials) reacts with the PMI of target product to produce 2-anilino-N-phenylsuccinimide (hereinafter also referred to as APSI), this APSI further reacts with maleic anhydride to produce N-(2,5-dioxo-1-phenyl-3-pyrrolidinyl)-N-phenylmaleamic acid (hereinafter also referred to as PPMA), and this PPMA further decomposes to produce PMI. It is also known that PMA is isomerized to produce N-phenylfumaramic acid (hereinafter also referred to as PFA). When PMI in which the intermediates and the by-products thereof are present as impurities is used, as one of the copolymerization components for obtaining the resin product as described above, there are problems that the obtained copolymer is colored or discolors, or the deterioration in qualities (heat resistance and strength) of the copolymer occurs by causing phenomena such as silver streak and fish eye, and the like.
In order to solve these problems, for example, Patent Literature 3 discloses a method of reducing acid components (PMA, maleic anhydride, and fumaric acid) in the maleimide by separating a reaction mixture after ring-closure imidization into the organic solvent layer containing the maleimide and the catalyst layer, subjecting the maleimide-containing organic solvent layer to the water washing treatment at a temperature of 70° C. or more, then separating the organic solvent layer and the aqueous layer. In addition, for example, Patent Literature 10 discloses a method for producing N-substituted maleimide having a purity of 95% by weight or more by adding a primary amine and maleic anhydride, in the presence of an organic solvent that can be azeotropically distilled with water and a supported catalyst where an organic phosphonic acid having a melting point of 150° C. or more and insoluble with the N-substituted maleimide is supported on a solid carrier, to carry out a dehydration ring closure reaction at 100 to 140° C.
Furthermore, for example, Patent Literature 11 discloses that, in the storage stabilization method that prevents color change by adjusting the contents of a primary amine and 2-amino-N-substituted succinimide compound in the maleimide compound to specific amounts or less, as a method of adjusting the amount of the 2-amino-N-substituted succinimide compound, the production of 2-amino-N-substituted succinimide compound can be suppressed by controlling the molar ratio of maleic anhydride and the primary amine during the reaction or by adding maleic anhydride in the latter half of the reaction to decompose the 2-amino-N-substituted succinimide compound.